The present invention is directed to an imaging device for a printing form, having at least one first laser diode bar and one second laser diode bar, the laser diodes on the first laser diode bar being disposed in a first line, and the laser diodes on the second laser diode bar being disposed in a second line, including a micro-optical array for generating aberration-corrected intermediate image spots of the laser diodes and a macro-optical imaging optics for generating image spots of the intermediate image spots on the printing form. The present invention is also directed to a method for arranging optical components in an imaging device for a printing form.
To an increasing degree, laser diode bars, which accommodate a number of laser diodes, particularly arranged in one line (linear array), are being used as light sources in imaging devices for printing forms, whether in a printing-form imagesetter or in a print unit of a printing press (direct on-press imaging print unit). An imaging device having such laser diode bars, in particular individually controllable laser diode bars, is described, for example, in U.S. Patent Application No. 2002/0005890 A1. Typically, the laser diode bars have widths on the order of centimeters and preferably accommodate between 30 and 80 emitters or laser diodes. The greater the number of laser diodes (whether integrated on a laser diode bar or distributed over a plurality of laser diode bars), the greater is also a temporal and spatial parallelization of the imaging of a printing form, it also being possible to suitably shorten the total exposure duration necessary for imaging the printing surface of the printing form. At the same time, however, it is essential for the functionality of the imaging device, particularly when an interleave imaging method is used in accordance with U.S. Patent Application 2002/0005890 A1, that all emitters on the laser diode bar be intact and also remain so for a longest possible service period. As the number of laser diodes on a laser diode bar goes up, the probability increases that a laser diode on a laser diode bar has failed or will fail. For that reason, a large number of laser diodes on a laser diode bar is disadvantageously associated with the danger of a rapid loss of operability.
At the same time, when assembling or mounting a laser diode bar in an imaging device, one encounters a particular difficulty that has implications for the imaging process: Although very stringent demands are placed on the positional tolerances of the emitters in the manufacturing of the laser diode bar, in particular to facilitate an interleave imaging process, this positional precision can be lost again during assembly. Unequal thermal expansion coefficients of the laser diode bar and of a holding element, such as a heat sink element, can cause the laser diode bar to become deformed during soldering. Frequently, the result of such a deformation is a tilted, distorted, or even curved characteristic of the line of laser diodes, which is, therefore, also referred to as a smile effect. The deviation in the emitter actual position from the emitter setpoint position, caused by the deformation, is often made even worse by a micro-optical array or micro-optics, i.e., by an array of optical elements which is assigned to the laser diode bar and in which individual optical elements (although sometimes also integrated in one component) only function in response to individual laser diodes. The larger the laser diode bar is, the greater is also the difference in expansion and/or difference in contraction induced by the temperature variation. For that reason, when a large laser diode bar is used, there is the inherent risk of a pronounced smile effect resulting from the assembly operation.
From the European Patent Application No. EP 0 641 116 A1, it is known that a micro-optical array in an imaging device can include individual microlenses, in each instance, one microlens being assigned to one laser diode bar on a laser diode bar. The microlenses have a common image plane. A macro-optical array, i.e., an array of optical elements, which acts simultaneously in response to the light from all laser diodes, projects the light emitted by the laser diode bars from the image plane onto a plane in which a photoreceptor is located. The optical axes of the microlenses coincide in each case with the optical axes of the laser diodes. For that reason, no correction is made in response to a possible deviation in the laser diodes' actual position from a setpoint position.
One possible way to compensate for the smile effect of a laser diode bar during imaging is described, for example, in U.S. Patent Application No. 2003/0026176 A1. A two-dimensional printing form surface is scanned by light beams from an imaging device having a number of laser diodes on a laser diode bar, rapidly in a first direction, and slowly in a second direction that is linearly independent, in particular orthogonal to the first direction. When, in response to simultaneous triggering, the image spots of the light beams do not lie on a desired curve, in particular straight line, printing dots can be produced on a projection line by the action of the light energy on the printing form surface in that the individual laser diodes are triggered with a time delay in such a way that one of the laser diodes emits light at the very moment when its image spot sweeps over the projection line. It is clear that this projection does, in fact, lead to an array of printing dots on the projection line, however there is no possible way to correct positional deviations in the direction orthogonal to the first direction in which a fast scanning takes place.